Most of the tips are the ones that you probably know. So, you can use the list as a checklist (which it normally is in my "customer notes sheet" : ) ).

Here are the tips:

1. Get A Fast & Less-Loaded Web Server

This is a must. Whatever you do to make your website faster, if it is served slow, there's nothing to do.

Make sure that the server your website is hosted has low ping values & it is not over-loaded. Specially if you've a database and/or using dynamic files (like asp, asp.net, php, ruby..) server over-load can kill the performance.

If you've a simple website with few visitors then shared hosting is ok. For a website getting popular, think of a VPS with guaranteed RAM & CPU usage. If the website is already popular, a dedicated server will be the best performing one.

2. Know When To Use GIF, JPG & PNG Files.

Playing with the quality of images will dramatically lower the sizes of them while they will still be looking good.

GIF is suitable for images with few colors like logos, text & line art. When saving a GIF file, make sure you use a small color pallette (learn more).

JPG is good for images with lots of colors & details like photographs. Decrease the quality of a JPG image before saving. It will still look good for a web image (learn more).

PNG, a format specially for websites, has great quality – both transparent & non-transparent – is specially functional when you're in need of quality transparent images. Don't forget that IE6 has problems in displaying them (learn more).

3. Use XHTML – No Tables

This is easy to tell but may be harder to implement for anyone that has few experience with XHTML.

The thing is, tables are rendered slower than DIVs by browsers. You can do more with less code & no need to say it is much better to be compatible with any browser (including mobile browsers), in means of SEO, etc..

Update: There may be cases when using tables are much more logical like "tabular data". You may have to code a lot for creating the look of a simple 5 columned table. Don't do that & use tables when needed.

What is tried to be mentioned here is, use XHTML rather than tables in general. But don't try to draw images with XHTML.

4. Use CSS Smartly & Compressed

Mention every style in CSS files.

Find the common properties in objects, define them once & try not to repeat them.

If using a significant amount of totally different properties for a specific page, define them in another CSS file & only include on that page. So users viewing other pages won't need to download those properties.

5. Merge & Compress JavaScript Files

If a JavaScript file will be used only in few pages, try to include them inside those pages only (like CSS in step 4).

Compress JavaScript files. It lowers the size seriously & doesn't have any bad effects on your side. This & this are some of the popular compressors.

6. Manage Included Files Better

There may be several files included in webpages like CSS files, JavaScript files and other JavaScript files for statistic services, social bookmarking sites, widgets & more.

Try to decrease the number of these files:

If using digg-reddit-like voting widgets, don't use them in stories that won't hit the frontpage & remove the ones that were submitted few days ago as they won't have any help after sometime.

Use only 1 statistic service if possible. Every web-based service requires new files to be installed & they are called each time a page is loaded.

Host files on your side & don't use web addresses as file paths.

If you're using any JS frameworks like jQuery or MooTools, don't call the .js files from their websites but download & upload them to your website.

Don't try to call any file from a web address like "http://.." as this always requires dns queries to be done & will work slower.

PHP Speedy (WRD post)is a very effective solution that does all the combining & compressing automatically. You may want to try it.

7. Use HTTP Compression

HTTP compression is one of the most effective step with a little effort.

Today, almost every browser & web server supports HTTP compression. Using it, your files are sent to the visitor compressed & un-compressed on their side & you can have up to 90% success for the text based files (like HTML).

You have 2 popular choices with this:

HTTP compression: a feature of the web server (can be enabled on the server side)

As you can guess, this is a huge subject. But there are some major things like:

Seperate the static & dynamic content. This will help writing less-code.

Create smart & effective functions. For ex: if you're going to use a user's name & birthdate at the same page, don't run 2 queries or functions, your function must call 2 of them. Simply, try to do everything once.

Connect to the database as few as you can & once connected, call only the data you need.

Try to keep database sizes small. If you've a large database, sometimes seperating it into several databases perform better, specially for the data which is reached not so often. From the server's point of view: 4-250mbs of dbs perform better than 1-1gb of db.

9. Cache Webpages

Caching is a superb way of decreasing the stress on the server by running more static files & less queries. It simply saves a static, HTML version of the page to be displayed & displays that file rather than the dynamic one.

For CMS applications like WordPress, there are already ready-to-use caching plugins (like WP-Cache). For PHP, here is a great article from SitePoint.

A small note, caching can also be done at the web server level. But if you're on a shared hosting account, you better be lucky to have it on.

What Do You Suggest?

Besides the tips mentioned above, what are your tips to make websites faster?